In wireless communication, signal fading is a key problem that affects transmission quality. Whether it is due to multipath effect, shadow effect or path loss, the signal will always face various interferences during the propagation process. However, the continuous development of anti-fading technology enables us to achieve high-quality communication in noisy wireless environments. From classic diversity reception, channel coding to modern adaptive modulation, MIMO technology, these innovations provide strong technical support for 5G, satellite communications and the Internet of Things. Understanding how these technologies shape future wireless communications is the key to exploring the next generation of networks!
The following will share three key anti-fading and anti-interference technologies based on Semtech’s LoRa wireless spread spectrum anti-interference technology: diversity reception technology, Rake receiver and signal combination method and multi-branch diversity reception and its adaptive technology.
1. Diversity reception technology:
Fading is the main obstacle to signal transmission in wireless communication channels. Fading is caused by multipath propagation. That is, signals from different paths can generate constructive or destructive interference with each other. Diversity reception is a solution to this problem.
Wireless channel diversity is a method of obtaining information from multiple signals transmitted through independent fading paths. It uses the randomness of radio propagation to find independent signal paths for communication. This is a very simple concept. If one path experiences deep fading, another independent path may have a strong signal. Since there are multiple paths to choose from, the receiver’s instantaneous and average SNR can be improved, but in general, the diversity decision is made by the receiver.
LoRa uses spread spectrum technology to combat multipath effects and fading, and increases the robustness of reception by extending the time characteristics of the signal, which is consistent with the goal of diversity reception technology. Although LoRa itself does not directly use spatial diversity or frequency diversity, it indirectly improves the system’s anti-interference ability through the unique design of signal characteristics in the time and frequency domains.
Rake Receiver and Signal Combining Methods:
What is a Rake Receiver? A Rake receiver is a technique designed to handle multipath propagation by coherently combining signals from different paths to improve the signal-to-noise ratio.
Rake receivers are used specifically in CDMA cellular systems to combine multipath components, which are delayed versions of the original signal transmission. This combination is done to improve the signal-to-noise ratio (SNR) at the receiver. Rake receivers attempt to collect time-shifted versions of the original signal by providing a separate correlator for each multipath signal. This is possible because when the relative propagation delay of the multipath components exceeds one chip period, they are effectively uncorrelated with each other. The design of a Rake receiver can be viewed as a series of delayed correlator taps fed from a common antenna.
If each correlator tap is delayed to match the arrival time of a particular transmitted signal, then the outputs of each tap can be recombined in phase. Once an RF signal with a particular propagation time is locked onto by a correlator tap, an estimate of the gain or loss experienced by that signal must be made. The weights of the taps perform this gain normalization function. Once adjusted, the outputs of each Rake tap can be combined to form a better version of the transmitted signal.
Although LoRa does not directly use Rake receivers, the essence of its spread spectrum technology can also be regarded as a generalized “signal combination method” because it can comprehensively utilize the energy in multipath signals and reduce the impact of fading.
3. Multi-branch diversity reception and its adaptive technology:
Multipath propagation and shadowing effects in wireless communications can cause random fluctuations in signal strength, which is called fading. Fading can significantly affect the performance of the communication link, resulting in an increase in the bit error rate (BER) and a decrease in channel capacity. Among them, diversity technology improves signal quality by receiving multiple independent signal copies and selects the branch with the best signal quality from multiple branches of the received signal. This technology is suitable for systems with low hardware complexity, but the performance improvement is limited.
Adaptive technology proposes to dynamically adjust the modulation mode and coding rate of the signal through adaptive modulation and coding (AMC). When the channel conditions are good, high-order modulation and less coding redundancy are used. When the channel conditions are poor, low-order modulation and more coding redundancy are used. This method effectively improves the channel capacity while maintaining a low bit error rate.
For LoRa technology, it is not multi-branch diversity reception in essence. It uses spread spectrum technology to improve anti-interference ability. Spread spectrum technology itself is an indirect form of diversity. By distributing signal energy over a wide frequency band, the interference effect of specific frequencies is reduced. If combined with multi-branch diversity reception, such as using multiple antennas to receive LoRa signals from different paths, the system performance can be further improved.
LoRa also uses adaptive data rate (ADR), which is an adaptive technology. ADR dynamically adjusts the data rate and transmit power according to channel conditions to optimize transmission distance and link reliability. When the signal is strong, LoRa can choose a higher data rate and a lower spreading factor (SF) to improve throughput. When the signal is weak or the distance is far, it chooses a lower data rate and a higher SF to increase anti-interference ability.
